What do telephony & error codes mean?

These are the most common error codes

401 Invalid credentials The SIP ID or SIP password is incorrect.
404 Number unknown The number doesn’t exist, isn’t assigned to a connection, or isn’t configured in the recipient’s device or phone system.
486 Busy The recipient’s line is currently busy.
487 Call ended Standard signal when the other party hangs up.

 

Other commonly transmitted codes

100 Trying Call setup in progress.
180 Ringing The recipient’s device is signaling an incoming call.
183 Session progress Connection is being established.
200 OK Call accepted or device registered.
301 Moved permanently Permanent call forwarding active.
302 Moved temporarily Temporary call forwarding active.
400 Bad request Incorrect parameters in the request.
403 Forbidden No permission to place this call.
407 Proxy authentication required Authentication needed for outbound calls.
408 Request timeout No response, possibly due to network issues.
480 Temporarily unavailable Recipient unavailable (e.g., DND active).
481 Call does not exist Status message not received.
482 Loop detected Call forwarding loop detected.
488 Not acceptable here Unsupported codec, encryption, or transmission method.
500 Server error General server issue.
502 Bad gateway Incorrect or faulty gateway.
503 Service unavailable SIP service temporarily down.
603 Declined Call rejected manually or automatically.
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